In conventional cellular radio systems, geographical areas are divided up into a plurality of adjoining cells, in which mobile stations within a cell communicate with a base transceiver station. The frequency band within which cellular radio systems operate is limited in band width, and so available carrier frequencies need to be reused in order to provide sufficient user capacity in the system. Carrier frequencies are reused from cell to cell, and in conventional systems it is usual to divide each nominally hexagonal cell into three sectors (a trisected cell) and to use omni-directional or sectorized antennas.
There is increased capacity demand for use of cellular radio systems. In deployments where the base transceiver stations at the center of cells have insufficient capacity to deal with demand from mobile stations within cells, in order to increase call carrying capacity it is required to reduce the size of cells and create more cells of smaller area. However, creation of new cells involves creation of new base transceiver stations which has the problem of increased equipment cost, and other associated costs such as the cost of renting or buying suitable sites. Further, it is increasingly difficult to obtain planning permission for new antenna sites. Thus, any techniques which allow increase of capacity at existing base transceiver stations are important.
The type of antenna used at the base station site can potentially make significant improvements to the range and capacity of a cellular radio system. In one approach a base station antenna pattern comprises a beam of narrow angular widths as shown in FIGS. 1 and 2 herein. A narrow radiation beam 1 is directed by a base station Smart antenna 2 at a desired mobile station 3. The beam is narrow in both an azimuth and elevation planes, and tracks the mobile's movements. When compared to an omni-directional antenna, such a narrow beam has dual benefits of having high gain, leading to increased range in a thermal noise limited environment, and of rejecting interference from co-channel reuse cells, due to spatial separation of beams, thereby allowing higher capacity in a cell without cell splitting. A narrow beam has an advantage of reducing interference in a balanced manner on an uplink and a downlink path.
Where each cell has a number of smart antennas having narrow beams which track individual mobiles, there results an overall reduction in carrier to interference (C/I) ratio due to the statistical probability that different beams reusing the same carrier frequency will be pointing in different directions, having different azimuths. The likelihood of two or more beams having a same carrier frequency intercepting each other is diminished. The narrower the beams, the lower the probability that a mobile will intercept a same frequency beam of a different cell in which the same carrier frequency is re-used. Although a narrow radiation beam is formed at radio frequencies typically in the 900 MHz, 1800 MHz or 1900 MHz bands, a narrow beam can usefully be visualized as analogous to search light beams which emanate from the base station, and track the mobiles. When contrasted with an omni-directional antenna, this creates a high quality transmission path with minimal interference. For the purposes of this document, the use of the word "omni-directional" is intended to convey the meaning of having radiation coverage over an area corresponding to substantially the whole geographic area of a cell. The extent of the advantage of a narrow beam antenna over an omni-directional antenna is a function of the beam width of the narrow beam antenna. The narrower the beam width, the greater the advantage.
However, the tracking beam antenna array, whilst providing improved carrier to interference ratio is vulnerable to fading, particularly since all elements in the antenna array may be closely spaced together, and may therefore all experience fading at the same time.
In another approach, there are provided a plurality of relatively narrow beams which are spatially fixed. As a mobile moves across an area covered by a plurality of beams, the mobile must be handed over from beam to beam, and using a smart antenna arrangement, frequencies can be switched between beams to follow a mobile, so that the mobile can remain communicating on the same carrier frequency without the need to hand over to a different carried frequency. However, the smart antenna arrangement required for a switched beam approach is also susceptible to fading, for the same reasons as the tracking beams as described above.
One solution used to partially overcome the effects of fading in conventional omni-directional antennas and sectorized antennas is to employ diversity.sup.1. Referring to FIG. 3 herein, there is shown an example of a coverage area for a cellular radio system divided into a plurality of nominal hexagonal cells, each cell being sectorized into nominal 120.degree. angular sectors. Examples of 120.degree. sectors are illustrated 300, 301, 302. Conventionally, each 120.degree. sector may employ diversity, and be served by a pair of antennas, comprising first and second antenna elements spaced apart from each other by a distance of the order of 2 to 3 meters or so. Such antenna pairs help to overcome Rayleigh fading. When one antenna is in fade, and receiving a weak signal, the other antenna of the pair may be out of fade and receiving a stronger signal. A deployment of conventional diversity pair antennas in a nominally hexagonal cell is illustrated schematically in FIGS. 4 and 5 herein. On an uplink, each antenna has a 120.degree. wide beam of high gain sensitivity, from which it picks up signals from mobile stations within a zone covered by the beam. Beams from the two antennas overlap each other, so that a signal transmitted by a mobile station MS within a zone covered by both beams will be received by both antennas. In a tri-sectorized cell using a diversity pair antenna approach, there may be mounted a triangular support 500 on a mast 501, on each side of the triangular support, there being positioned a pair of antennas 502, 503; 504, 505; and 506, 507. A conventional diversity pair antenna arrangement comprises a main antenna 502 and a diverse antenna 503, the two antennas connected to a diversity receiver. If the antennas of an antenna pair are spaced far enough apart, any fading experienced by one antenna of the pair will be largely uncorrelated with fading experienced by the other antenna of the pair. The gain advantage which can be achieved from employing conventional diversity pair antennas is typically within the range 3 dB to about 8 dB. The conventional tri-sectorized base station antenna configuration produces an increase in carrier to interference ratio over a cellular radio system employing diverse omni-directional antennas, whilst retaining an improved signal to noise ratio by use of diversity pair antennas in each sector.
Referring to FIG. 6 herein, there is illustrated schematically a switching arrangement of a prior art antenna arrangement for a tri-sectorized cell incorporating three 120.degree. sectors. Each sector is provided with a diversity antenna pair 600, 601, 602 respectively, each antenna comprising a main antenna and a diversity antenna. Each antenna is connected to an input of an RF switch 603 controlled by diversity radio receiver 604, which scans the received signals present on each of the diversity antenna pairs of the three sectors. Sector scanning is performed to determine when to hand over a mobile station from one sector to another. The receiver 604 receiving an RF signal from a mobile on one sector occasionally scans the antennas of the other sectors to compare the received signals strengths on those adjacent sectors, to determine when to hand over a call between sectors. An example of a prior art diversity receiver having sector scanning facility is the NT-800-DR dual mode radio unit available from Northern Telecom Limited.
Whereas antennas having broad 120.degree. sector coverage may provide some increase in system capacity through frequency reuse, for further improving the carrier to interference ratio and hence system capacity use of multiple narrow beams in a sector, is desirable. To produce a plurality of narrow directional beams, an antenna array having plurality of antenna elements is used. The relative phase and amplitude weighting of each of the elements determine the direction and width of the beams. The antenna array is ideally as compact as possible, in order to reduce size and weight. However because such small apertures are used, the antennas are vulnerable to fading. The size and shape of array antennas which provide a narrow beam operation with up to seven or eight beams per 120.degree. sector means that these antennas can experience fading on all beams simultaneously due to the close spacing of elements of the array antenna.
Whilst it is desirable to use a large number of narrow beams in a sector to increase capacity and it is desirable to use compact aperture antennas from the advantage of low weight and small size, there remains a fading problem in all beams in a sector simultaneously using such a configuration.
Previous solutions to fading using diversity pair antennas all have wide aperture antennas. There remains a problem of increasing system capacity by greater re-use of carrier frequencies in a narrow beam approach whilst overcoming fading in all beams of an antenna.